German authorities are investigating explosive new allegations of a police cover-up over Anis Amri, the Berlin Christmas market attacker.

The Berlin regional government has filed criminal charges against two of its own police officers alleging they falsified documents to cover up a missed opportunity to arrest Amri more than a month before the attack.

Twelve people were killed and more than 60 injured when Amri drove a lorry into a packed Christmas market last December.

According to the new allegations police had the evidence to arrest Amri for drug dealing more than six weeks earlier, but did not act.

Surveillance of Amri’s telephone calls, which had been ordered over fears he was a potential terror threat, uncovered evidence he was involved in large-scale drug dealing.

Previous police reports indicated than Amri was only a minor drug dealer, trading in small amounts that would not justify an arrest or secure a conviction.

But Andreas Geisel, interior minister for the Berlin regional government, said he had uncovered an official surveillance report which showed police had evidence Amri was much more heavily involved in the drugs trade.

“In this document, Anis Amri is accused of professional and organised drugs trafficking,” Mr Geisel said.

“It would have served as the basis of an arrest warrant that could have prevented the attack.”

But there was a second version of the report in which Amri’s involvement was downplayed. “This version no longer refers to professional and organised trafficking, but only to drug dealing,” Mr Geisel said.

Crucially, the minister said the second version of the report was drawn up on Januray 17 — almost a month after the attacks — but “clearly backdated to November 1”.

Mr Geisel has filed criminal charges against two officers in the Landeskriminalamt, the Berlin police equivalent of CID.

There has already been public anger that police and intelligence agencies were unable to identify the threat from Amri, a rejected Tunisian asylum-seeker, despite a trail of evidence linking him to extremist circles and tip-offs from informants that he had talked of carrying out an attack.

But if it now transpires Berlin police missed the chance to take him off the streets six weeks before the attack it will raise serious questions.

“This is a new dimension. It would be devastating if a chance for an arrest was missed,” Burkard Dregger, a high-profile regional MP from Angela Merkel’s Christian Democrats (CDU) said.

“It’s a dramatic turnaround. We’ll have to discuss the consequences,” Frank Zimmerman of the rival Social Democrats (SPD) said